First, write a balanced chemical equation or the reaction:

  Na\(_2\)CO\(_3\) +  2HCl →  2NaCl  + H\(_2\)O +  CO\(_2\)

From the balanced equation, we can deduce that N\(_A\) = 2 , N\(_B\) = 1

We were also given from the question, C\(_A\) = ?  C\(_B\) = 0.2 mol/dm\(^3\)  V\(_A\) = 20cm\(^3\)  V\(_B\) = 25 cm\(^3\)

Using the relationship, \(\frac{C_A  V_A}{C_B V_B}\) = \(\frac{N_A}{N_B}\)

C\(_A\) = \(\frac{C_B V_B N_A}{V_A N_B}\)

C\(_A\) = \(\frac{0.2 X 25 X 2}{20 X 1}\)  =  0.5 mol/dm\(^3\)